A ball screw in which balls are capable of rolling between a screw shaft and a nut enables a reduction in coefficient of friction in rotating the screw shaft with respect to the nut, as compared with a screw coming into sliding contact. Hence, the ball screw has been utilized in various fields such as a positioning mechanism of a machine tool, feeding mechanism, steering gear of an automobile, and other fields.
Nuts of ball screws are classified into various types such as a return-pipe type, deflector type, and end-cap type, according to how balls are made to circulate. The return-pipe type of nut, which is most commonly used, uses a return pipe for circulating balls. In this nut, the balls are scooped from a nut shaft by the return pipe, pass inside the return pipe, and return to an original position to circulate. The deflector type of nut is compact in size. In this nut, the balls are changed in proceeding direction by a deflector, climb over an outer circumferential surface of a screw shaft, and return to an original position to circulate. The end-cap type of nut is a nut suitable for high-speed feeding. Specifically, the balls are scooped by an end-cap, pass through a through hole of the nut, and return to an original position to circulate. The end-cap type of ball screw is disclosed in the following Patent Document 1. Patent Document 1: International Publication WO2003/021133